TL;DR: You pull the mechanisms that move the laser head out of a CD/DVD drive, put two of these at a 90 degrees angle, and voila: XY-platform with a very small build area, but high positional accuracy!

Using the laser head positioning mechanism from a CD drive to build a highly accurate XY platform is not a new idea, of course. Urs at Hackteria credits Peter Jansen on the RepRap Builders list, who got it from his friend Trevyn, who for all whe know may have gotten it from somewhere else: